1. Field of the Invention
The present invention pertains to metal fabricating, and more particularly to apparatus that bends and stretches metal blanks.
2. Description of the Related Art
Various equipment has been developed to deep draw flat blanks of metal into three-dimensional pieces. The drawing process involves controlled flowing of material as the blank is forced over a punch of desired size and shape during a draw stroke. The blank is held around its outer margins between a draw ring and a blank holder as the blank central region is forced over the punch to from the finished piece.
To rigidly hold the blank in place during the forming process, it is known to employ draw beads in the draw ring and blank holder. The draw beads provide a very large gripping force on the blank margins such that they do not slip in the draw ring or blank holder during the draw stroke. Examples of prior equipment with draw beads may be seen in U.S. Pat. Nos. 5,507,189 and 5,644,943, and in U.S. published patent application 2002/0095968. The foregoing patents and published patent application show complementary male and female components that are integral with the draw ring and blank holder. In other prior equipment, the draw beads consist of loose male members in one or other of the draw ring or blank holder, which are complementary to recesses in the other of the draw ring or blank holder.
In many instances, the blank was formed into a piece having a continuous wall, a domed end, and a flange extending outwardly from the wall. After the draw stroke was complete, the formed piece was removed from the draw ring and blank holder. The wall was often cut to remove the flange, which was discarded. The finished piece thus consisted of the wall and the domed end integral with the wall.
It is also know to employ a stationary step and ledge design to tightly grip the blank margins only at the end of the draw stroke. FIGS. 1–3 show a prior draw ring 1 with a step 3. A stationary punch 5 has a mating ledge 7. As the draw ring 1, blank holder 9, and blank 11 near the end of the draw stroke, arrow 12, the blank material is formed at reference numeral 13 into a double bend between the draw ring step 3 and the punch ledge 7. The double bend 13 tightly grips the blank 11 at the completion of the draw stroke. After the draw stroke is completed, the piece wall 15 is cut along the plane 17. The step and ledge design is sometimes used in combination with the draw beads discussed previously.
A common problem involving deep drawn pieces with continuous walls was that the wall free edges tended to spring outwardly after the flanges were cut away. The spring back occurred whether the draw beads, the step and ledge, or the combination of the draw beads and step and ledge were used to grip the blank.
For many types of pieces, the spring back was tolerable. On the other hand, some pieces could not tolerate any spring back. For example, a certain prior automotive fuel tank was manufactured by matching the free edges of two formed pieces and welding the two free edges together to make the tank. However, the spring back of the two pieces made it very difficult to align their free edges and weld them. Consequently, the scrap rate was unacceptably high.
Thus, a need exists for improvements in the process for deep drawings metal blanks.